Non-compliant multilayered balloon for a catheter

ABSTRACT

Balloon catheter comprises an elongate catheter shaft having a proximal section, a distal section, and an inflation lumen, and a multilayer balloon on the distal section of the shaft. The multilayer balloon comprises at least a first layer and a second layer having a combined wall thickness and an outer-most layer. The first layer is made of a first polymer material having a first maximum blow-up-ratio. The second layer is made of a second polymer material having a second maximum blow-up-ratio greater than the first maximum blow-up-ratio and the second layer is an inner layer relative to the first layer. The at least first and second layers define a compliance less than that of a single layer balloon made of the first polymer material with a wall thickness equal to the combined wall thickness. The outer-most layer is made of a third polymer material.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/897,202, filed Oct. 4, 2010, now U.S. Pat. No. 8,394,055, which is acontinuation of U.S. application Ser. No. 11/313,041, filed Dec. 20,2005 (issued as U.S. Pat. No. 7,828,766 on Nov. 9, 2010), the contentsof each of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The invention relates to the field of intravascular medical devices, andmore particularly to a balloon for a catheter.

In percutaneous transluminal coronary angioplasty (PTCA) procedures, aguiding catheter is advanced until the distal tip of the guidingcatheter is seated in the ostium of a desired coronary artery. Aguidewire, positioned within an inner lumen of an dilatation catheter,is first advanced out of the distal end of the guiding catheter into thepatient's coronary artery until the distal end of the guidewire crossesa lesion to be dilated. Then the dilatation catheter having aninflatable balloon on the distal portion thereof is advanced into thepatient's coronary anatomy, over the previously introduced guidewire,until the balloon of the dilatation catheter is properly positionedacross the lesion. Once properly positioned, the dilatation balloon isinflated with liquid one or more times to a predetermined size atrelatively high pressures (e.g. greater than 8 atmospheres) so that thestenosis is compressed against the arterial wall and the wall expandedto open up the passageway. Generally, the inflated diameter of theballoon is approximately the same diameter as the native diameter of thebody lumen being dilated so as to complete the dilatation but notoverexpand the artery wall. Substantial, uncontrolled expansion of theballoon against the vessel wall can cause trauma to the vessel wall.After the balloon is finally deflated, blood flow resumes through thedilated artery and the dilatation catheter can be removed therefrom.

In such angioplasty procedures, there may be restenosis of the artery,i.e. reformation of the arterial blockage, which necessitates eitheranother angioplasty procedure, or some other method of repairing orstrengthening the dilated area. To reduce the restenosis rate and tostrengthen the dilated area, physicians frequently implant anintravascular prosthesis, generally called a stent, inside the artery atthe site of the lesion. Stents may also be used to repair vessels havingan intimal flap or dissection or to generally strengthen a weakenedsection of a vessel. Stents are usually delivered to a desired locationwithin a coronary artery in a contracted condition on a balloon of acatheter which is similar in many respects to a balloon angioplastycatheter, and expanded to a larger diameter by expansion of the balloon.The balloon is deflated to remove the catheter and the stent left inplace within the artery at the site of the dilated lesion.

Catheter balloons are typically manufactured independently of thecatheter shaft and then secured to the catheter shaft with an adhesiveor other bonding method. In standard balloon manufacture, a polymer tubeis blown biaxially under the action of axial tension, internal pressure,and heat within a mold. The polymer tube may either be simultaneouslystretched in the radial and axial directions, or sequentially, by firststretching axially and then radially. The starting dimensions of thepolymer tube and the finished dimensions of the blow-molded balloonwithin the mold are a measure of the degree to which the polymericmaterial has been stretched and oriented during balloon blowing, andaffect important characteristics of the finished balloon such as rupturepressure and compliance. The blow-up-ratio (BUR), namely, the ratio ofthe outer diameter of the blown balloon (i.e., the mold inner diameter)to the inner diameter of the polymer tube, is a measure of thosedimensions. Beyond a critical BUR for a given polymer, the balloonblowing process becomes unstable and the polymer tubing often rupturesor tears before a balloon is fully formed.

In the standard blow molding process, an initiated bubble rapidly growsin diameter until it is constrained by the mold wall. The hoop stress inthe wall of the tubing, as it grows into a balloon, may be approximatedby the expression:σ_(h)=(P·R)/δwhere P is the inflation pressure, R is the mean radius of the polymerictube at any time during the inflation and δ, delta, is the wallthickness of the tubing. For a balloon to be initiated from the tubing,the inflation pressure should be such that the wall hoop stress exceedsthe material resistance (typically the yield stress) to stretching atthe blowing temperature. Once a balloon is initiated from the tubing, itgrows rapidly in size until it touches the mold wall. As the balloongrows, the radius increases and the balloon wall thickness decreases.This results in a rapid increase in the wall hoop stress during constantpressure blowing. If the wall hoop stress of the growing balloon exceedsthe ultimate hoop strength of the material, rupture will occur. As aresult, there is a limit to the BUR (i.e., a maximum attainable BUR) ofa polymeric material forming the balloon layer(s).

In the design of catheter balloons, balloon characteristics such asstrength, flexibility and compliance must be tailored to provide optimalperformance for a particular application. Angioplasty and stent deliveryballoons preferably have high strength for inflation at relatively highpressure, and high flexibility and softness for improved ability totrack the tortuous anatomy and cross lesions. The balloon compliance,which depends on factors such as the nature of the balloon material, theballoon wall thickness, and processing conditions, is chosen so that theballoon will have a desired amount of expansion during inflation.Compliant balloons, for example balloons made from materials such aspolyethylene, exhibit substantial stretching upon the application oftensile force. Noncompliant balloons, for example balloons made frommaterials such as PET, exhibit relatively little stretching duringinflation, and therefore provide controlled radial growth in response toan increase in inflation pressure within the working pressure range.However, noncompliant balloons generally have relatively low flexibilityand softness, so that it has been difficult to provide a low compliantballoon with high flexibility and softness for enhanced cathetertrackability. A balance is typically struck between the competingconsiderations of softness/flexibility and noncompliance, which, as aresult, has limited the degree to which the compliance of catheterballoons can be further lowered.

Therefore, what has been needed is a catheter balloon with very lowcompliance, yet with excellent ability to track within the patient'svasculature and cross lesions therein. The present invention satisfiesthese and other needs.

SUMMARY OF THE INVENTION

The invention is directed to a balloon catheter having a multilayeredballoon which has a first layer and at least a second layer, and whichhas noncompliant limited radial expansion beyond the nominal diameter ofthe balloon. By selecting the polymeric materials forming the balloonlayers, and arranging and radially expanding the multiple layers of theballoon in accordance with the invention, a balloon is provided havingan improved low compliance, preferably in combination with highflexibility and softness.

A multilayered balloon of the invention is preferably formed in whole orin part of coextruded polymeric tubular layers, and provides for ease ofmanufacture of the balloon and balloon catheter formed therefrom. Themultilayered balloon is typically formed by conventional blow-molding inwhich a multilayered polymeric tube is radially expanded within aballoon mold. The resulting multilayered balloon has an inflated shapewhich corresponds to the inner surface of the mold and which has adiameter about equal to the inner diameter of the balloon mold, commonlyreferred to as the balloon's nominal working diameter. The nominalpressure is the inflation pressure required to fill the balloon to thenominal working diameter. In accordance with the invention, the balloonexpands a very small amount (i.e., noncompliantly) at pressures abovethe nominal pressure. As a result, the balloon minimizes injury to apatient's blood vessel, which can otherwise occur if the ballooncontinues to expand a substantial uncontrolled amount at increasinginflation pressures above nominal.

As discussed above, the blow-up-ratio (BUR) of the balloon formed from apolymer tube should be understood to refer to the ratio of the outerdiameter of the blown balloon expanded within the mold (i.e., the moldinner diameter) to the inner diameter of the polymer tube prior to beingexpanded in the mold. Each individual layer of the multilayered balloonsimilarly has its own BUR based on the ratio of the inner diameter ofthe mold and the inner diameter (prior to expansion in the mold) of thelayer of the polymeric tube. For a given balloon wall thickness, therupture strength generally increases and the radial compliance decreasesas the balloon BUR increases. For standard pressure driven blow moldingof catheter balloons, typical BURs range from about 4.5 to about 8.0depending on the material and the product application.

A method of making a balloon of the invention increases the amount ofballoon material that is highly oriented in the radial direction, toprovide a balloon with limited radial expansion at increasing inflationpressures (i.e., to provide a noncompliant balloon). Specifically, amultilayered balloon of the invention has polymeric materials that canbe expanded to higher BURs as the inner layer(s) of the balloon, whilelower BUR materials are the outer layer(s) of the balloon. In apresently preferred embodiment, the balloon has a first layer of a firstpolymeric material and a second layer of a second polymeric materialwhich has a lower Shore durometer hardness than the first polymericmaterial and which can be expanded during balloon blowing to a higherBUR (without rupturing or tearing) than the higher Shore durometerhardness material of the first layer, and the second layer is an innerlayer relative to the first layer. For example, one embodiment, themultilayered balloon inner layer is formed of a polyether block amide(PEBA) material (e.g., commercially available as PEBAX®) having a Shoredurometer hardness of about 60-70D while the outer layer is formed of aPEBA material having a higher Shore durometer hardness of about 70-72D.However, a variety of suitable materials can be used including materialswhich are of the same material classification/family, or differentclasses of materials. The multilayered balloon generally has two or morelayers (i.e., layers formed of materials which differ in some respectsuch as different Shore durometer hardnesses), although it typicallydoes not have more than five layers.

Despite presence of the lower durometer material forming the second(inner) layer of the multilayered balloon, a first embodiment of theinvention provides a balloon which has a very low compliance. Forexample, a balloon of the invention having a first (outer) layer of afirst durometer, and one or more inner layer(s) of successively lowerdurometers (i.e., increasingly softer materials), has a lower compliancethan a balloon having about the same wall thickness but formed of 100%of the highest durometer material (i.e., the material forming theouter-most layer of the balloon of the invention). Compared to a balloonformed of 100% of the highest durometer material, a balloon of theinvention has effectively replaced a part of the balloon wall thicknesswith the layer(s) of lower durometer (softer) material(s), which wouldtypically be expected to increase the compliance. While not wishing tobe bound by theory, it is believed that the balloon provides thenoncompliant behavior through the specific combination of highlyoriented layers of the balloon, and particularly by maximizing theorientation of the inner layer(s) of the balloon. The inner layerorientation significantly affects compliance of the balloon. Byselecting and arranging different materials that can be blown todifferent BURs in accordance with the invention, the balloon has layerswith successively increasing BURs from the outer to the inner layer(s),such that the BUR of each layer is preferably maximized and the innerlayer(s) have particularly high BURs. The layers of the balloon aretherefore optimized for compliance purposes. Although additional layersmay be added to the balloon, to, for example, increase the total wallthickness to a desired value, the arrangement of the basic layers inaccordance with the invention cannot be varied without resulting in ahigher compliance balloon.

Additionally, the invention can alternatively provide for a balloon witha low compliance but with very thin walls. For example, one embodimentis directed to a multilayered balloon having a first (outer) layer of afirst durometer material and one or more inner layer(s) of successivelylower durometer materials which has a compliance not substantiallygreater than (e.g., not more than about 10% to about 20% greater than),and preferably about equal to a balloon which is formed of 100% of thehighest durometer material but which has a larger wall thickness thanthe multilayered balloon of the invention. The embodiment of the balloonhaving a very thin total wall thickness provides an improved low profileand flexibility due to the thinner walls of the balloon, but, inaccordance with the invention, nonetheless continues to provide a lowcompliance despite the thin wall.

The rupture pressure and compliance of a balloon are affected by thestrength (e.g., hoop strength) of a balloon. Because a softer materialgenerally has a relatively lower hoop strength, the presence of thelower durometer material forming the inner layer(s) of the balloon isnot generally expected to provide a relatively higher modulus balloon.However, a multilayered balloon of the invention preferably has a highermodulus than, and a rupture pressure which is not substantially lessthan, a balloon formed of 100% of the highest durometer material.

The presence of the lower durometer material inner layer(s) does providelayers of increased softness, and therefore preferably provides aballoon that is softer and more flexible than a balloon formed of 100%of the highest durometer material.

Prior multilayered balloons with layers of polymers having differentstrengths/softnesses typically arrange the layers so that the durometerhardnesses decreased from the inner to the outer layer, for variousballoon design considerations. For example, lower durometer (softer)materials are typically preferred as outer layers for designconsiderations such as pinhole resistance, stent retention, and thelike. In contrast, a balloon of the invention arranges layers so thatthe highest durometer material has on an inner surface thereof a layerof a lower durometer material, and configures the layers to provide fora maximized BUR which produces an improved combination ofcharacteristics including a very low compliance. However, with the innerlayer(s) of the balloon of the invention optimized for compliancepurposes as discussed above, one embodiment of a balloon of theinvention has an outer-most layer of a relatively soft material, to, forexample, enhance stent retention.

The compliance of the balloon should be understood to refer to thedegree to which the polymeric wall of the balloon stretches/distends asthe balloon expands beyond the nominal diameter of the balloon. Thecompliance curve expresses the balloon outer diameter as a function ofincreasing inflation pressure in millimeters/atmospheres (mm/atm), sothat a steeper curve or section of the curve indicates a highercompliance than a flatter curve. The term “noncompliant”, should beunderstood to mean a balloon with compliance of not greater than about0.03 mm/atm, preferably not greater than about 0.025 mm/atm. Incontrast, compliant balloons typically have a compliance of greater thanabout 0.045 mm/atm. A noncompliant balloon of the invention generallyhas a compliance above nominal of about 0.01 to about 0.02 mm/atm, for a3.0 mm diameter balloon. The compliance of the balloon is typicallyabout 25% to about 50% less than the compliance of a balloon with asimilar wall thickness but made from 100% of the first (e.g., highestdurometer) material.

In a presently preferred embodiment, the polymeric material of the firstlayer and the polymeric material of the second layer of the multilayeredballoon are elastomers, which typically have a lower flexural modulusthan nonelastomers. Elastomeric polymers suitable for forming the firstand/or second layer of the multilayered balloon typically have aflexural modulus of about 40 kpsi to about 110 kpsi. Thus, unlikenonelastomeric materials such as PET which have been used in the past toprovide relatively low compliance catheter balloons, the multilayerednoncompliant balloon of the invention is preferably formed of one ormore elastomers which provide for improved balloon flexibility.

One aspect of the invention is directed to a method of making anoncompliant multilayered balloon for catheter. The method generallycomprises selecting a first and a second polymeric material, the secondpolymeric material having been determined to have a higher maximumattainable BUR than the first polymeric material, and forming amultilayered tube having a first layer of the first polymeric material,and a second layer of the second polymeric material wherein the secondlayer is an inner layer relative to the first layer. The maximumattainable BUR of a polymeric material is typically determinedexperimentally, although characteristics such as the ultimate tensilestrength and elongation to break of the material maybe indicative atleast for some materials (e.g., a material having a relatively higherultimate tensile strength and elongation to break is expected, ingeneral, to have a higher maximum BUR). The inner diameter of each layerof the multilayered tube is selected so that the ratio of the innerdiameter of the balloon mold and the inner diameter of the layer of themultilayered tube (prior to being radially expanded in the balloon mold)is substantially at a maximum blow-up-ratio for the polymeric materialforming the layer. Thus, the method includes forming the blow-moldedmultilayered balloon by radially expanding the multilayered tube in amold, so that radially expanding the tube to the mold inner diameterradially expands each layer substantially to the maximum blow-up-ratioof the polymeric material forming the layer, such that the multilayeredballoon has a lower compliance above the nominal working diameter than aballoon consisting of the first elastomeric polymeric material.

Various designs for balloon catheters well known in the art may be usedin the catheter system of the invention. For example, conventionalover-the-wire balloon catheters for angioplasty or stent deliveryusually include a guidewire receiving lumen extending the length of thecatheter shaft from a guidewire proximal port in the proximal end of theshaft to a guidewire distal port in the catheter distal end. Rapidexchange balloon catheters for similar procedures generally include arelatively short guidewire lumen extending from a guidewire port locateddistal to the proximal end of the shaft to the catheter distal end.

The multilayered balloon of the invention provides a very low compliancefor controlled balloon expansion, without compromising relatively highflexibility and softness for excellent ability to track the patient'svasculature and cross lesions. As a result, the balloon catheter of theinvention has improved performance due to the flexibility, softness, andcontrolled expansion of the balloon. The balloon provides the surprisingresult of a very low compliance from the addition of the lower durometer(softer) second material. Consequently, a multilayered balloon of theinvention will provide a much lower compliance than a balloon with thesame wall thickness but made from just the higher durometer (stiffer)material, or will provide a much thinner walled balloon but without theexpected increase in compliance. These and other advantages of theinvention will become more apparent from the following detaileddescription of the invention and the accompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational view, partially in section, of an over-the-wiretype stent delivery balloon catheter embodying features of theinvention.

FIGS. 2 and 3 are transverse cross sectional views of the catheter ofFIG. 1, taken along lines 2-2 and 3-3, respectively.

FIG. 4 illustrates the balloon catheter of FIG. 1 with the ballooninflated.

FIG. 5 is a longitudinal cross sectional view of multilayered balloontubing in a balloon mold prior to being radially expanded therein, in amethod embodying features of the invention.

FIG. 6 is graphical compliance data, with balloon diameter measured inmillimeters as the ordinate and inflation pressure measured inatmospheres as the abscissa, comparing a multilayered balloon of theinvention with a single layered balloon formed of 100% of the highestdurometer material.

FIG. 7 is graphical modulus data, with balloon modulus in kpsi as theordinate and inflation pressure measured in atmospheres as the abscissa,comparing a multilayered balloon of the invention with a single layeredballoon formed of 100% of the highest durometer material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a stent delivery balloon catheter 10 which embodiesfeatures of the invention, generally comprising an elongated cathetershaft 11 having a proximal shaft section 12, a distal shaft section 13,an inflation lumen 21, and a guidewire lumen 22 configured to slidablyreceive a guidewire 23 therein, and having a balloon 14 mounted on thedistal shaft section. An adapter 17 on a proximal end of the cathetershaft provides access to the guidewire lumen 22, and has an arm 24configured for connecting to a source of inflation fluid (not shown).FIG. 1 illustrates the balloon in a noninflated configuration foradvancement within a patient's body lumen 18. A radially expandablestent 16 is releasably mounted on the balloon 14 for delivery anddeployment within the body lumen 18. The balloon catheter 10 is advancedin the body lumen 18 with the balloon 14 in the noninflatedconfiguration, and the balloon inflated by introducing inflation fluidinto the balloon interior to expand the balloon 14 and stent 16 mountedthereon. FIG. 4 illustrates the balloon catheter 10 with the balloon inthe inflated configuration to expand the stent against the wall of thebody lumen 18. The balloon 14 is then deflated to allow forrepositioning or removal of the catheter from the body lumen 18, leavingthe stent 16 implanted in the body lumen 18.

In the illustrated embodiment, the shaft comprises an outer tubularmember 19 defining the inflation lumen 21, and an inner tubular member20 defining the guidewire lumen 22 and positioned in the outer tubularmember 19 such that the inflation lumen 21 is the annular space betweenthe inner surface of the outer tubular member 19 and the outer surfaceof the inner tubular member 20, as best shown in FIG. 2 illustrating atransverse cross section of the catheter of FIG. 1, taken along line2-2. The balloon 14 has a proximal skirt section sealingly secured tothe distal end of the outer tubular member 19, and a distal skirtsection sealingly secured to a distal end of the inner tubular member20, so that an interior 15 of the balloon is in fluid communication withthe inflation lumen 21 of the shaft. FIG. 3 illustrates a transversecross section of the catheter of FIG. 1, taken along line 3-3, althoughthe space between the inner surface of the noninflated balloon and theouter surface of the portion of the shaft 11 therein is somewhatexaggerated in FIGS. 1 and 3, for ease of illustration. A variety ofalternative suitable catheter shaft configurations can be used as areconventionally known.

Although not illustrated, the balloon 14 of the invention typically hasa noninflated configuration with wings wrapped around the balloon toform a low profile configuration for introduction and advancement withina patient's body lumen. As a result, the balloon inflates to a nominalworking diameter by unfolding and filling the molded volume of theballoon.

Balloon 14 has a first layer 30, and a second layer 31 which is an innerlayer relative to the first layer 30. In the illustrated embodiment, thesecond layer 31 is on an inner surface of the first layer 30, with thefirst layer 30 defining an outer surface of the balloon 14 and thesecond layer 31 defining an inner surface of the balloon 14. However,the balloon 14 of the invention can alternatively have one or moreadditional layers (not shown). Additional layer(s) increase thedimensions of the tube/balloon formed therefrom to a desired value,and/or can be used to provide an inner or outer surface of the balloonwith a desired characteristic. Therefore, it should be understood thatthe balloon 14 of the invention discussed below has at least two layers,and optionally includes one or more additional layers, unless otherwisenoted as having a specified set number of layers.

The first (outer) layer 30 is formed of a first polymeric material, andthe second (inner) layer 31 is formed of a second polymeric materialthat can be expanded to a higher BUR than the first polymeric material.The second (inner) layer 31 is at a BUR which is typically about 15% toabout 40% greater than the BUR of the first (outer) layer 30. Each layer30, 31 is preferably at its maximum BUR, so that the balloon has layersof highly oriented material and, consequently, a very low compliance.

A variety of suitable materials can be used to form the first and secondlayers 30, 31, including polyamides, polyurethanes, and polyesters. In apresently preferred embodiment, the first and second polymeric materialsare elastomers providing a relatively low flexural modulus for balloonflexibility, although nonelastomers can alternatively be used. Presentlypreferred materials are from the same polymeric family/class such aspolyamides including nylons and polyether block amides (PEBAX). Formingthe layers of compatible polymeric materials allows for heat fusionbonding the layers together. The layers can alternatively be formed ofdifferent polymer classes which are not sufficiently compatible tofusion bond together, in which case a tie layer is typically providedbetween the outer and inner layers 30, 31 to bond the balloon layerstogether. For example, a PET inner layer and a PEBAX typically have atie layer of an adhesive polymer such as Primacor (a functionalizedpolyolefin) therebetween.

The balloon 14 is formed by a method in which the layers of materialthat can be expanded to higher BURs are the inner layers of the balloontubing, and lower BUR materials are the outer layers, and the balloon isblow-molded such that each layer is optimized for radial orientation.The resulting balloon has an increased resistance to radial expansion atincreasing inflation pressures.

The balloon 14 is blow-molded from a multilayered tube which has thefirst layer 30, and the second layer 31 as an inner layer relative tothe first layer 30. However, as discussed above, a balloon of theinvention may have one or more additional layers, so that the tubingused to blow-mold the balloon would similarly be formed with theadditional layer(s). The tube is typically formed by coextrusion,although a variety of suitable method may be used. For example, in oneembodiment, a multilayered tube is formed by coextruding at least twolayers, and one or more additional layers are added to the coextrudedtube for example by heat shrinking, dip coating, adhesive or fusionbonding, or frictionally engaging the additional layer(s) to thecoextruded tube.

The multilayered tube is then radially expanded in a balloon mold toform the balloon 14. FIG. 5 illustrates the multilayered tube 40 in aballoon mold 41 having an interior chamber 42 with a shape configured toform the balloon 14, and an inner diameter about equal to the nominalworking diameter of the expanded balloon 14. The multilayered tube 40 istypically stretched axially and heated during blow molding in theballoon mold, as is conventionally known. For example, in oneembodiment, the tube is longitudinally stretched by about 200% duringblow molding, which produces a biaxially oriented balloon. The singlewall thickness of the tube (prior to being radially expanded in themold) is about 0.1 to about 0.4 mm, and the single wall thickness of theresulting balloon (radially expanded in the mold) is about 0.01 to about0.04 mm, depending on the desired balloon characteristics and uses.

The materials and dimensions of the multilayered tube 40 and balloonmold 41 are selected so that each layer of the resulting balloon hasbeen radially expanded to substantially its maximum possible amount,expressed as the BUR of the balloon layers. In a presently preferredembodiment, the outer layer 30 has a higher Shore durometer hardness andtherefore lower elongation than the one or more inner layers. Theelongation of each layer is typically about 10% to about 50%, and morespecifically about 20% more than the elongation of the outer layerimmediately adjacent thereto.

In a presently preferred embodiment, the first (outer) layer 30 is aPEBAX having a Shore durometer hardness of about 72D, and the second(inner) layer 31 is a PEBAX having a Shore durometer hardness of about63D. The PEBAX 72D outer layer 30 typically has a BUR of between about 6and 7, and the PEBAX 63D inner layer 31 a BUR of between about 7 and 8.

In one embodiment, a mid layer (not shown) of intermediate BUR and/ordurometer hardness is provided between the outer and inner layers 30,31. For example, in one presently preferred embodiment, the balloon 14has a first, outer layer 30 of PEBAX 72D, a second, inner layer 31 ofPEBAX 63D, and a midlayer (not shown) therebetween of PEBAX 70D. In apresently preferred embodiment, the inner and mid layers have a smallerwall thickness than the highest durometer layer therearound, andtypically together make up about 5% to about 15% of the total wallthickness of the multilayered balloon. The balloon 14 can similarly haveone or more additional layers (not shown) which similarly continue thepattern of sequentially increasing BUR and/or durometer from the innertoward the outer layers of the balloon. However, in one embodiment, theballoon 14 has a relatively soft outer-most layer (not shown) having aShore durometer hardness less than the immediately adjacent inner layerof the balloon, which facilitates embedding the stent 16 into the outersurface of the balloon for improved stent retention. Such a relativelysoft outer-most layer typically has of a relatively low Shore durometerhardness of about 40D to about 55D.

The multilayered balloon of the invention has a low compliance, and arelatively high rupture pressure, particularly when compared to aballoon of otherwise similar construction but formed solely of thehighest durometer material used to make the multilayered balloon of theinvention (e.g., a 72D PEBAX outer layer of multilayered balloon 14), orcompared to a balloon formed of layers of different durometer materialsbut not layered in accordance with the invention. The compliance istypically determined for the pressure range extending from the nominalpressure (i.e., the pressure required to fill the molded volume of theballoon to the blow-molded nominal diameter) to the burst pressure orthe rated burst pressure of the balloon. The rated burst pressure (RBP),calculated from the average rupture pressure, is the pressure at which99.9% of the balloons can be pressurized to without rupturing, with 95%confidence.

The multilayered balloon 14 has a nominal pressure of about 6 to about12 atm, and more typically of about 7 to about 9 atm, and a RBP of about14 to about 22 atms, more typically about 18 to about 20 atms. Therupture pressure is typically about equal to, greater than, or notsubstantially less than (i.e., not more than about 5% to about 15% lessthan) a rupture pressure of a balloon of otherwise similar constructionbut formed solely of the highest durometer material.

In one embodiment, a multilayered balloon of the invention having atleast a 72D PEBAX outer layer and a 63D PEBAX inner layer reaches thenominal diameter of the balloon at about 8 to about 9 atm, andthereafter stretches in a noncompliant manner with a compliance of about0.01 to about 0.02 mm/atm within the working pressure range (e.g., 8-20atm) of the multilayered balloon to a diameter which is not more thanabout 8% greater than the nominal diameter.

Due to the presence of the softer durometer inner layer(s), the flexuralmodulus of a multilayered balloon of the invention is expected generallyto be about 90% to about 95% of the flexural modulus of a balloonconsisting of the first (e.g., higher durometer) elastomeric polymericmaterial of the layer 30.

Example

Multilayered balloon tubing, formed by coextrusion, had overalldimensions of 0.0155 inch inner diameter (ID) and 0.0365 inch outerdiameter (OD). The tubing had an inner layer of 63D PEBAX with a wallthickness 0.001 inches, a midlayer of 70D PEBAX with a wall thickness of0.001 inches, and an outer layer of 72D PEBAX with a wall thickness of0.0085 inches. Wall thickness values are a single wall thickness, unlessotherwise identified as a double wall thickness (DWT). The tubing wasblow-molded by heating and pressurizing the tubing in a 0.1215 inch IDballoon mold in a single blow cycle, resulting in a multilayered balloonhaving an average wall thickness (DWT) of 0.00163 inches and thefollowing BURs for the balloon layers: 63D Inner Layer ID of 0.0155 inchgives a BUR of 7.83 (0.1215/0.0155); 70D midlayer ID of 0.0175 inchgives a BUR of 6.94 (0.1215/0.0175); and 72D outer layer ID of 0.0195inch gives a BUR of 6.23 (0.1215/0.0195). The calculated BUR value ofballoons may vary slightly depending on whether the ID of the mold orthe OD of the balloon at blow is used for the calculation. The resultingmultilayered balloon had overall dimensions of about 0.1214 inch ID and0.1230 inch OD.

The compliance and modulus of the multilayered balloon were compared toa comparison balloon similarly formed and with approximately the samewall thickness but from a single layer (100%) of the 72D PEBAX. Thecomparison balloon was blow-molded in a 0.1250 inch ID balloon mold,using balloon tubing extruded to a 0.0190 inch ID and a 0.0365 inch OD,to form a balloon having the desired wall thickness. The resultingballoon had an average wall thickness of 0.00165 inches and a BUR of6.58 (0.1250/0.0190). The multilayered balloon of the invention and thecomparison monolithic balloon each had a nominal pressure of about 8atm, and a burst pressure of greater than 20 atm, and more specifically,an average rupture pressure of about 25 atm. The compliance curves ofthe multilayered balloon and the comparison monolithic balloon are shownin FIG. 6, and are generated by inflating a balloon subassembly andmeasuring the change in the balloon outer diameter in response toincreasing inflation pressures.

As illustrated in FIG. 6, the compliance from nominal (8 atm) to 20 atmis about 0.018 mm/atm for the multilayered balloon of the invention,compared to about 0.028 mm/atm for the monolithic comparison balloon.Thus, despite the presence of the lower durometer material mid and innerlayers, such that the 72D PEBAX made up a smaller percentage of the wallthickness of the balloon than in the monolithic balloon made solely of72D PEBAX, the multilayered balloon of the invention had a lowercompliance. Specifically, the outer layer of PEBAX 72D made up about 87%of the wall thickness of the multilayered balloon, compared to 100% ofthe monolithic balloon. Similarly, FIG. 7 illustrates the incrementalmodulus comparison (modulus value from P_(n) to P_(n+1)) of a trilayeredPebax 63D/70D/72D balloon of the invention and a monolithic Pebax 72Dcomparison balloon. The modulus of the multilayered balloon of theinvention, illustrated graphically in FIG. 7, is higher than the modulusof the monolithic comparison balloon. The modulus values are derivedfrom the compliance curve data, and are specifically determined from theequationE=((P _(n+1) D _(n+1))/DWT_(n+1)−(P _(n) D _(n))/DWT_(n))/(D _(n+1) −D_(n))/D _(n)where E is modulus, P is inflation pressure, D is diameter, and DWT isdouble wall thickness.

The BUR of the 72D PEBAX outer layer of the trilayer balloon of theinvention is less than the BUR of the monolithic 72D PEBAX balloon.However, the multilayered balloon of the invention facilitates expandingthe lower durometer inner layers to relatively high BURs, and provides aballoon with an overall BUR that is relatively high. The inner and midlayers are at relatively high BURs of about 7 to about 8, and preferablyare at higher BURs than are possible if attempting to use the sameblow-molding procedure to form a similar balloon but from 100% of thematerial of either the inner or the mid layer. For example, PEBAX 63Dextruded to form tubing having an ID of 0.0195 inches and an OD of0.0355 inches can not be blown into a 0.118 inch ID balloon mold (i.e.,a BUR of 6) in a single blow cycle without rupturing during theblow-molding process.

The absolute average wall thickness of the multilayered balloon in theabove Example was about equal to the wall thickness of the monolithicballoon, allowing for a direct comparison of the compliance and modulusof the balloons. However, it should be understood that the wallthickness of the multilayered balloon of the invention couldalternatively have been made less, so that the compliance and moduluscomparisons would have been based on normalized wall thicknesses.

The dimensions of catheter 10 are determined largely by the size of theballoon and guidewire to be employed, the catheter type, and the size ofthe artery or other body lumen through which the catheter must pass orthe size of the stent being delivered. Typically, the outer tubularmember 19 has an outer diameter of about 0.025 to about 0.04 inch (0.064to 0.10 cm), usually about 0.037 inch (0.094 cm), and the wall thicknessof the outer tubular member 19 can vary from about 0.002 to about 0.008inch (0.0051 to 0.02 cm), typically about 0.003 to 0.005 inch (0.0076 to0.013 cm). The inner tubular member 20 typically has an inner diameterof about 0.01 to about 0.018 inch (0.025 to 0.046 cm), usually about0.016 inch (0.04 cm), and a wall thickness of about 0.004 to about 0.008inch (0.01 to 0.02 cm). The overall length of the catheter 10 may rangefrom about 100 to about 150 cm, and is typically about 143 cm.Preferably, balloon 14 has a length about 0.8 cm to about 6 cm, and aninflated working diameter of about 2 to about 5 mm.

The various components may be joined using conventional bonding methodssuch as by fusion bonding or use of adhesives. Although the shaft isillustrated as having an inner and outer tubular member, a variety ofsuitable shaft configurations may be used including a dual lumenextruded shaft having a side-by-side lumens extruded therein. Similarly,although the embodiment illustrated in FIG. 1 is an over-the-wire typestent delivery balloon catheter, the catheter of this invention maycomprise a variety of intravascular catheters, such as a rapid exchangetype balloon catheter. Rapid exchange catheters generally comprise ashaft having a relatively short guidewire lumen extending from aguidewire distal port at the catheter distal end to a guidewire proximalport spaced a relatively short distance from the distal end of thecatheter and a relatively large distance from the proximal end of thecatheter.

While the present invention is described herein in terms of certainpreferred embodiments, those skilled in the art will recognize thatvarious modifications and improvements may be made to the inventionwithout departing from the scope thereof. Moreover, although individualfeatures of one embodiment of the invention may be discussed herein orshown in the drawings of the one embodiment and not in otherembodiments, it should be apparent that individual features of oneembodiment may be combined with one or more features of anotherembodiment or features from a plurality of embodiments.

The invention claimed is:
 1. A balloon catheter, comprising: an elongatecatheter shaft having a proximal section, a distal section, and aninflation lumen; and a multilayer balloon on the distal section of theelongate catheter shaft, the multilayer balloon comprising: at least afirst layer and a second layer having a combined wall thickness; thefirst layer made of a first polymer material having a first maximumblow-up-ratio; the second layer made of a second polymer material havinga second maximum blow-up-ratio greater than the first maximumblow-up-ratio, wherein the second layer is an inner layer relative tothe first layer; the at least first and second layers defining acompliance less than that of a single layer balloon made of the firstpolymer material with a wall thickness equal to the combined wallthickness; and an outer-most layer made of a third polymer material. 2.The balloon catheter of claim 1, wherein the first layer has a firstShore durometer hardness and the second layer has a second Shoredurometer hardness lower than the first Shore durometer hardness.
 3. Theballoon catheter of claim 2, wherein the third polymer layer has a thirdShore durometer hardness lower than the second Shore durometer hardness.4. The balloon catheter of claim 3, wherein the first Shore durometerhardness is about 70D to about 72D.
 5. The balloon catheter of claim 3,wherein the second Shore durometer hardness is about 60D to about 70D.6. The balloon catheter of claim 3, wherein the third Shore durometerhardness is about 40D to about 55D.
 7. The balloon catheter of claim 3,wherein the first Shore durometer hardness is about 70D to about 72D,the second Shore durometer hardness is about 60D to about 70D, and thethird Shore durometer hardness is about 40D to about 55D.
 8. The ballooncatheter of claim 1, wherein the third polymer material has third Shoredurometer hardness less than that of a polymer material of animmediately adjacent inner layer of the multilayer balloon.
 9. Theballoon catheter of claim 1, wherein the multilayer balloon has anominal working diameter corresponding to an inner mold diameter used toform the multilayer balloon, and at least the second layer issubstantially at the second maximum blow-up-ratio when the multilayerballoon is substantially at the nominal working diameter.
 10. Theballoon catheter of claim 9, wherein the first layer is substantially atthe first maximum blow-up-ratio when the multilayer balloon issubstantially at the nominal working diameter.
 11. The balloon catheterof claim 1, wherein the second layer defines an inner surface of themultilayer balloon.
 12. The balloon catheter of claim 1, furthercomprising an expandable stent mounted on the outer-most layer of themultilayer balloon.
 13. The balloon catheter of claim 1, wherein thefirst layer, the second layer, and the outer-most layer are coextruded.14. A multilayer balloon for a catheter comprising: at least a firstlayer and a second layer having a combined wall thickness; the firstlayer made of a first polymer material having a first maximumblow-up-ratio; the second layer made of a second polymer material havinga second maximum blow-up-ratio greater than the first maximumblow-up-ratio, wherein the second layer is an inner layer relative tothe first layer; the at least first and second layers defining acompliance less than that of a single layer balloon made of the firstpolymer material with a wall thickness equal to the combined wallthickness; and an outer-most layer made of a third polymer material. 15.The multilayer balloon for a catheter of claim 14, wherein the firstlayer has a first Shore durometer hardness and the second layer has asecond Shore durometer hardness lower than the first Shore durometerhardness.
 16. The multilayer balloon for a catheter of claim 15, whereinthe third polymer layer has a third Shore durometer hardness lower thanthe second Shore durometer hardness.
 17. The multilayer balloon for acatheter of claim 16, wherein the third Shore durometer hardness isabout 40D to about 55D.
 18. The multilayer balloon for a catheter ofclaim 16, wherein the first Shore durometer hardness is about 70D toabout 72D, the second Shore durometer hardness is about 60D to about70D, and the third Shore durometer hardness is about 40D to about 55D.19. The multilayer balloon for a catheter of claim 14, wherein the thirdpolymer material has third Shore durometer hardness less than that of apolymer material of an immediately adjacent inner layer of the balloon.20. The multilayer balloon for a catheter of claim 14, wherein themultilayer balloon has a nominal working diameter corresponding to aninner mold diameter used to form the multilayer balloon, and at leastthe second layer is substantially at the second maximum blow-up-ratiowhen the multilayer balloon is substantially at the nominal workingdiameter.
 21. The multilayer balloon for a catheter of claim 14, whereinthe first layer, the second layer, and the outer-most layer arecoextruded.